The present invention relates to a deep-hole gundrill that is especially suitable for machining materials that produce long chips.
Deep-hole gundrills, also referred to as gundrills below, are known in a wide range of designs from the prior art. Reference is made to VDI Standards 3210 and 3208 and DE 10 2012 016 660 A1 of the applicant, for example. In general, there is always an effort to improve the capability, machining accuracy, and process reliability of deep-hole drills. The latter requires a short chip in order to ensure that the chips are transported out of the bore. In addition, the cutting force or the energy required for drilling should be minimized in order to reduce the load on the drill shank and DE 10 2012 016 660 A1, deep-hole gundrills and the special characteristics thereof are described. In particular, paragraphs [0002] to [0012] of DE 10 2012 016 660 A1 are hereby incorporated into the present application by reference.
As already known from DE 10 2012 016 660 A1, the rake angle of deep-hole gundrills is typically 0°. Deep-hole gundrills having a rake angle of >0° are also known from the prior art. One advantage of a rake angle of 0° is that said rake angle is simple to produce by grinding. However, comparatively high cutting forces occur during machining. As a result, the bore can laterally deviate. This lateral deviation is also called “bore deviation”.
In addition, the possibility of influencing the forming of chips and therefore also the chip length is limited. When materials that produce long chips, such as high-quality and therefore also tough steel, are machined, this leads to problems especially at high feed rates and therefore limits productivity. This applies particularly to bores having a high ratio of length to diameter.
If deep bores are intended to be produced in materials that produce long chips, it is in accordance with the prior art to provide chip splitters and/or chip formers on the tool in order to minimize the chip size and thus ensure reliable transport of chips out of the bore. Chip splitters have an interrupted or curved blade. This has the disadvantage of expensive reflects the cutting edge, the bottom of the bore has an interrupted and/or curved shape when deep-hole gundrills having a chip splitter are used. This is often undesired.
Chip formers generally cause a rise in the feed forces and cutting forces, which can negatively affect the bore deviation.
Therefore, the present invention addresses the problem of providing a deep-hole gundrill that produces a short chip and therefore ensures process reliability even at high feed rates. In addition, the forces and torques acting on the drill head during machining are intended to be small so that the deviation of the bore is minimized and a long tool life is ensured.
Furthermore, the drive power required for drilling is intended to be low and the deep-hole gundrill according to the invention is intended to be relatively simple to produce and to regrind.
Said problem is solved by a deep-hole gundrill having the features of claim 1.
The chip forming surface according to the invention is flat but has two different plateaus with respect to the rake surface. The first section, which lies radially further inside, has a plateau lying further above the rake surface than the second section, which is formed radially further outside on the drill head. This design of the chip forming surface according to the invention has several positive effects:
The stepped chip forming surface according to the invention is relatively simple to produce by grinding, because the first section and the second section are flat faces that extend adjacently to each other in the direction of the rotational axis. If desired, said two sections can even be produced in one grinding process by means of a contoured grinding disk.
Furthermore, the step between the rake surface and the outer second section of the chip forming surface is relatively low, and therefore the chip produced by the outer cutting edge in this radially outer region is slightly raised on said step. The drive power required for this purpose is relatively low, even though the second section is arranged radially outside. Consequently, the torsional and bending loads resulting therefrom and the required drive power of the deep-hole gundrill according to the invention are also relatively low. This has a positive effect on the straightness of the bore and the feed rate.
The step between the rake surface and the first section, which is arranged radially further inside, is considerably higher, and therefore considerable deformation work is performed on the chip here, which deformation work causes the chip to break. Nevertheless, the torque resulting therefrom is relatively low, because the first section is located in close proximity to the rotational axis of the drill head.
The plateaus of the chip forming surface in the first section and the second section, which plateaus are offset from each other and from the rake surface, additionally cause the chip to be deflected toward the second wall of the flute. When the chip hits said wall, the chip is plastically deformed again and therefore finally breaks into the desired short pieces.
This results in a very short-breaking chip, a low cutting force, and high capability of the deep-hole gundrill according to the invention. In addition, the deep-hole gundrill according to the invention is relatively simple to produce. Another reason for this is that the chip forming surface consists of flat sections arranged adjacently to each other. In addition, the rake surface is flat and is positioned in close proximity to or even directly at the center of the drill head. This facilitates not only the grinding of the rake surface but also the grinding of a contoured tip of the drill head.
In an advantageous embodiment of the invention, it is provided that the second section lies at the height of the rake surface or slightly higher than the rake surface. Specifically, the outer part of the chip is then only slightly raised behind the blade, and this requires only small forces.
According to the invention, the two sections of the chip forming surface are designed as flat surfaces extending adjacently to each other, which simplifies the production.
It has proven advantageous if the distance H1 of the first section from the rake surface is more than twice as large as the distance H2 of the second section from the rake surface. A ratio of 4:1 has proven to be an effective compromise between a low cutting force or low drive power and, at the same time, high deformation forces acting on the chip. Consequently, the chip breaks so as to be short and nevertheless the risk of overloading of the deep-hole gundrill or of premature wear is minimized.
It has also proven to be advantageous if the rake angle of the rake surface equals 0° and the rake surface of the drill head extends through the rotational axis of the drill head or slightly below the rotational axis. In other words, the rake surface can be arranged at or slightly below the center of the drill head. For example, in the case of a deep-hole gundrill having a drill diameter of 6 mm, the rake surface can lie up to 2/100 mm below the center. In comparison with deep-hole gundrills having a chip breaker, the rake surface lies considerably closer to or at the center in the case of the deep-hole gundrill according to the invention.
In the case of deep-hole gundrills having a chip breaker that are known from the prior art, the rake surface on the outer cutting edge lies considerably below the center. The rake surface on the inner cutting edge lies in the region of the rotational axis at the center. Such cutting-edge geometry is usually considerably more expensive to produce and more difficult to regrind than the cutting-edge geometry according to the invention.
The tip of the deep-hole gundrill according to the invention can be designed in accordance with the requirements of the customer within a wide range. Because the rake surface lies at the center or only slightly below the center, customer-specific geometries of the bottom of the bore can be realized easily and precisely. This is a further advantage of the deep-hole gundrill according to the invention. An angle W is less than 90° so that the chip slides easily over a step between the first section and the second section without jamming. The smaller the angle W, the more easily the chip slides over said step.
It has also proven advantageous if the first diameter D1, which defines the transition between the first section and the second section, is approximately equal to half of the diameter of the drill head. In the case of this division of the chip forming surface, a good compromise has been achieved between large deformation work of the chip and, at the same time, low drive power.
Likewise in the case of the deep-hole gundrill according to the invention, a chip breaker can be formed on the rake surface in a manner known per se. Said chip breaker can be designed, for example, as a small chamfer, the front end of said chamfer being positioned somewhat (approximately 1/100 mm to 2/100 mm) below the center of the drill head.
In a manner known per se, the deep-hole gundrill according to the invention has a shank, the flute being formed in the drill head and at least in part in the shank.
The drill head according to the invention advantageously consists of carbide and, if necessary, is coated. The shank having the driver can also consist of carbide, the tool then being a solid carbide tool. Alternatively, the shank, having the flute, and a clamping sleeve can consist of a different material, such as steel.
Further advantages and advantageous embodiments of the invention can be found in the drawings below, the description thereof, and the claims (for protection).
All features described in the drawings, in the description of the drawings and in the claims can be essential to the invention in isolation and in any combination with each other.